Constant speed drives have been proposed for applications where it is desired to drive a generator at constant speed using a variable speed prime mover so that constant frequency electrical power is developed by the generator. Recently, advances in power electronics and control systems have resulted in the feasibility of electrically compensated constant speed drives (ECCSD) which utilize permanent magnet machines and a power converter in the speed compensation link of the drive. Such a drive is disclosed in Dishner et al. U.S. Pat. No. 4,695,776 (Sundstrand Docket No. B02150A-AT1-USA), assigned to the assignee of the instant application and the disclosure of which is hereby incorporated by reference.
It has been found that the permanent magnet machines in the speed compensation link of an ECCSD may be used to start the prime mover. In Baker et al. U.S. Pat. No. 4,697,090 (Sundstrand Docket No. B02277-AT1-USA), assigned to the assignee of the instant application and the disclosure of which is hereby incorporated by reference, an ECCSD is disclosed in which a differential speed summer includes a first input shaft coupled to an output shaft of a prime mover, a second input shaft coupled to an output shaft of a speed compensating permanent magnet machine and an output shaft coupled to a generator. A control permanent magnet machine includes a motive power shaft which is coupled to the output shaft of the prime mover or the output shaft of the differential. A power converter interconnects the electrical power windings of the permanent magnet machines. The system further includes means operable in a starting mode for operating the speed-compensating machine to cause the differential output shaft to rotate at increasing speeds. Once the synchronous speed of the generator is reached, power is applied to the output windings of the generator to cause the generator to operate as a motor. Thereafter, the speed compensating permanent magnet machine is operated to develop torque equal in magnitude to the torque developed by the generator. Starting torque is thus provided to the first input shaft of the differential to accelerate the prime mover to self-sustaining speed. Thereafter, the system operates in a generating mode so that constant frequency power is generated.
During operation in the starting mode, the torque provided by the speed-compensating permanent magnet machine may be developed by placing an electrical load thereon. In one embodiment, electrical power from the speed-compensating machine is provided to the control machine, which in turn develops additional starting torque which is delivered to the prime mover.
It has been found in this system that the speed-compensating machine must operate in the starting mode between zero speed and a multiple of full generator synchronous speed, and must operate in the generating mode in a speed range which is significantly less than its speed range in the starting mode. Thus the speed-compensating machine must operate over widely separated speed ranges. Further, the control permanent magnet machine operates in the starting mode up to a speed which is approximately 50% of its maximum speed during the generating mode. Thus, for a given machine power, the control permanent magnet machine must develop a large torque magnitude during operation in the starting mode.
A consequence of the foregoing is that the permanent magnet machines must be sized to accommodate the wide speed range differences and torque requirements in the starting and normal modes of operation. Thus, the machines are relatively large and heavy. This may prove to be a disadvantage in installations where small size and light weight are important, such as an aircraft or spacecraft.